1. Field of the Invention
This invention relates to a method of holding a wafer on an electrostatic chuck, a method of removing a wafer from an electrostatic chuck and an electrostatic chucking device which are used properly for etching systems, CVD systems, sputtering systems, etc. used widely for thin film forming and its patterning in fine processes represented by those for manufacturing semiconductor and other electronic devices.
2. Description of the Related Art
Conventionally, a semiconductor manufacturing system such as etching system, CVD system, sputtering system, etc. has been provided with an electrostatic chuck for fixing a wafer at a specified position so that the wafer can be processed in the manufacturing system. Such electrostatic chuck comprises a dielectric body formed by an insulator, and an electrode formed by a conductor for causing dielectric polarization in the dielectric body. The electrode is connected to a high voltage DC power supply. When a DC voltage is impressed to the electrode by this high voltage DC power supply, the dielectric body is dielectrically polarized, with which the dielectric body exhibits an attracting force. In other words, this electrostatic chuck can attract and hold the wafer on its dielectric body by the attracting force provided by impressing the DC voltage.
When removing the wafer from the electrostatic chuck after a desired processing, a voltage with reverse polarity is impressed to the electrostatic chuck so as to discharge the electric charge accumulated on the electrostatic chuck. Or the wafer is exposed to plasma to discharge the electric charge accumulated on the wafer.
When various plasma processings are carried out using such an electrostatic chuck, the temperature of the electrostatic chuck rises usually due to generated plasma. Furthermore, as in the case of an etching process, where a series of processings are carried out in a plurality of steps, the temperature of the wafer must be changed significantly between steps, and accordingly the temperature of the electrostatic chuck must also be changed significantly sometimes.
If the temperature of the electrostatic chuck is thus changed significantly during a processing, however, the following problem arises.
As a material for the dielectric body of the electrostatic chuck polyimide or ceramics is generally used, but polyimide is not suitable for use in high temperature. Thus, ceramics is usually used in processings at high temperatures such as plasma processing. As for ceramics, its specific resistance becomes low when the processing temperature is high as shown in FIG. 1. When the processing temperature goes low, its specific resistance increases. The specific resistance changes such way according to the processing temperature, and accordingly the attracting force of the electrostatic chuck also changes according to the processing temperature. When the processing temperature goes high, therefore, the attracting force becomes strong and when the processing temperature goes low, the attracting force becomes weak as shown in FIG. 2.
Consequently, when the temperature of the electrostatic chuck changes to which a fixed voltage is impressed constantly, the attracting force of the electrostatic chuck, that is, the wafer holding force of the electrostatic chuck also changes. If the wafer holding force is reduced at this time, problems such as wafer displacement may occur, and accordingly the processing may be failed. When the wafer holding force increases, the wafer becomes unremovable from the electrostatic chuck sometimes when it must be removed.